Neural modulation applications generally rely on leads that include contacts that are disposed in a fixed position within the lead body. The leads can be implanted at a location in a region of interest in a patient's body and surgically stabilized at the location. Typically, the region of interest can include neural elements, some of which are intended to be affected by stimulation (“desired elements”) and other neural elements (“undesired elements”) that, when inadvertently stimulated, can cause unwanted effects. The desired elements and undesired elements can be located in close proximity so that the stimulation, although intended for the desired elements, can spread to affect the undesired elements.
Once the lead is surgically stabilized in the location, scar tissue can forms over the lead. Accordingly, much of the programming of the electrical lead is done after the surgery is completed and after the patient has healed from the surgical procedure. Several programming strategies exist to decrease the stimulation of the undesired elements. Such strategies include adding multiple (and smaller) electrical contacts (electrodes) to the lead body so that the site(s) of neural tissue receiving electrical stimulation can be changed by deactivating electrode contacts that produce side effects and activating electrode contacts that predominantly produce the intended effects. In addition, it is possible to change other stimulation parameters such as the polarity, frequency, and/or pulse width. In addition, other technological modifications include “current steering” and directional electrical contacts. Current steering includes fractionating the current, as well as selectively delivering the current in a particular direction using “directional” deep brain stimulation (“DBS”) leads. Directional DBS leads have electrical contacts that partially cover one segment of a cylindrical lead, aimed at favoring the effects of stimulation to one side of the lead while sparing the other side.
With respect to spinal cord stimulation or cortical stimulation, a few millimeters of difference in the contact positioning over the spinal cord or cortex (epidurally or subdurally) can affect which neural elements are stimulated. Thus far, this limitation has been addressed by designing leads with more contacts or increasing the density of contacts on the lead body. An issue with high density leads, such as paddle leads with 16 or 32 electrical contacts (particularly relating to epidural stimulation), is that the cushion of cerebrospinal fluid (CSF) between the lead and the spinal cord or cortex “washes” or “blurs” the precision of such higher density leads. For example, the short distance between the several small electrical contacts of such leads can result in no difference from one electrical contact to the other effectively resulting in stimulation of the desired and undesired neural elements. This is the case because in order to achieve effective spinal cord or brain cortical stimulation, the voltage of the electrical current has to be ramped to a point that the sphere of stimulation is so large that minor changes in electrical contact position do not effectively alter which neural elements of the spinal cord will be affected. In addition, the cerebrospinal fluid cushion is highly conductive, further washing out minor differences is electrical fields generated by small adjacent electrical contacts.
With respect to deep brain stimulation, there are several possible applications already in practice and under investigation, including deep brain stimulation for Alzheimer's disease, stroke rehabilitation, depression, obsessive-compulsive disorder, epilepsy, tremor and Parkinson's disease. Lead location is critically associated with outcome. Because deep brain structures are often small and in tight proximity with other structures, it is important to favor stimulation towards the desired structures only. While directional leads can help with that (as well as current steering), little can be done to change the orientation of the directional leads once the implant procedure is completed and the patient has healed.
While these programming strategies each have some utility, they each have limitations. The best scenario remains implanting the lead so that the contacts are in an ideal position for stimulation, eliminating the need for such programming strategies. However, little can be done to change the orientation of the contacts once the patient has healed from the implantation procedure. Further, it would be desirable to have larger contacts implanted in the ideal location rather than the small contacts built into high density leads.